As shown in FIG. 1, earth bolts may protect electrical/electronic components of a vehicle from electromagnetic shock and also suppress noise therefrom. In fact, the earth bolts are safety-related parts of a vehicle, and thus, electro-conductivity and anti-rusting property of the earth bolts should be maintained. In conventional techniques, typical earth bolts may have a surface treatment layer to improve electro-conductivity and anti-rusting property, as shown in FIG. 2. For example, the conventional surface treatment layer may be formed by plating with zinc-nickel including high concentration of nickel of about 12 to 18 wt % and applying a black chromate film layer thereon.
Although such zinc-nickel plated layer has good corrosion resistance, its electro-conductivity may be inferior. Thus, when a problem, for example, poor removal of vehicle body electroplating, noncompliance of fastening torque and deprived formation of earth bolts, occurs, an engine may be damaged due to poor grounding of the zinc-nickel plated layer as illustrated in FIG. 3.
Therefore, some efforts have been made to solve the technical difficulties described above. For example, a method for coating a zinc-plated steel sheet with a chromate film and a silicon-based coating solution has been reported. However, when the earth bolt was applied to a vehicle engine room, improvement of electro-conductivity was not sufficient. Thus, it has been desired to develop a surface treatment method of an earth bolt and an earth bolt manufactured therefrom, to thereby improve reliability of a vehicle body by protecting the electrical/electronic components in the vehicle from electromagnetic shock, removing noise, and providing significantly enhancing corrosion resistance and electro-conductivity.
The above information disclosed in this Background section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.